PACIFICA, Calif. — Two slick mile-long tunnels are undergoing final safety tests this month, poised to divert motorists away from an ocean cliff-hanging roadway dubbed Devil’s Slide south of San Francisco to a smooth, Alpine-like passageway unlike any other in the United States.

The $439 million project, paid with federal emergency funds, features massive exhaust fans, carbon monoxide sensors and a pair of 1,000-foot bridges soaring 125 feet above a grassy horse ranch. A series of 10 fireproof shelters are staggered between the double bores, and remote cameras dangle from the ceiling, monitored by an around-the-clock safety staff of 15.

The tunnels, the first in the United States designed and built with an Austrian technique, have a Euro-glossiness to them, with white, glistening walls and shiny pipes gliding down a rounded ceiling. There’s a bit of theme park vibe as well, with retaining walls and fake boulders at the entrance sculpted by the man who shaped and molded Disneyland’s Indiana Jones ride.

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The Tom Lantos Tunnels, named after the late congressman, are the first tunnels built in California in more than 50 years. There are only a handful of tunnels under construction in the United States, including the Alaskan Way Tunnel in Seattle, and the fourth bore of the Caldecott Tunnel, just 34 miles east of Devil’s Slide in the eastern San Francisco Bay area.

Unlike those tunnels built to relieve commuter congestion, this new pair, 15 miles south of San Francisco, will divert a treacherous 1.2-mile stretch of the Pacific Coast Highway that constantly erodes and frequently collapses.

Just three years after its 1937 completion, the road tumbled into pounding waves below. The road has fallen eight times since, causing costly closures that have devastated communities to the south that depend on the route for daily commutes and for tourism from motorists heading south from San Francisco.

In addition to slides, every year there are serious, often deadly, accidents on the narrow roadway, which twists so sharply that drivers are forced to slow to less than 25 miles per hour.